This invention relates to apparatus and methods for controlling tape drive apparatus and more particularly relates to reel-to-reel tape drive apparatus controlled by a host system such that speed of the tape and/or the length of the gap between blocks of data vary adaptively according to conditions sensed while the tape is moving so the tape drive's operating characteristics will best utilize the operational mode and availability of the host.
Tape drive apparatus heretofore proposed conventionally operates to advance the tape at one speed or at one of a plurality of operator selectable speeds. In data processing operations, high speed movement of the tape is efficient for certain types of applications such as archiving, interchange and disaster processing. In such applications, a new read or write instruction can be expected while the IBG on the tape is passing the read/write heads; i.e., in sufficient time before the next block of data is to be processed. Movement of tape at lower speeds is more efficient for other applications, such as journaling and normal processing, where data is read or written intermittently. The speed is therefore slow enough to permit the tape to stop at a point within the IBG from which, upon receipt of a delayed read or write instruction, it can be accelerated to a preselected low speed before the next block of data passes the read/write head.
If tape speed is excessive for the particular type of application being performed, a "backhitch" will occur because a new read or write command will not be received while the tape at the head is within the IBG. During a backhitch, the tape is brought to a stop downstream of the overrun IBG, then driven in a reverse direction and caused to come to rest at a point upstream of said IBG. The tape will remain stationary until a read or write instruction is received; whereupon it must accelerate as that IBG passes the read or write head, so that the tape will be at proper speed as it passes said head.
Assuming that a tape is driven at 100 ips (inches per second), a delay of over 1 second could occur as a result of each backhitch; whereas if that tape is driven at 12.5 ips, permitting the tape to stop within the IBG, the delay would merely be about 48 milliseconds, but the data rate would be considerably lower. Thus, whether operation at a particular higher speed or a particular lower speed is more efficient is a function of the lengths of the data blocks and the number thereof that can be processed without a backhitch; or, more properly, the ratio of the number of successive data blocks processed to the number of backhitches, for data blocks of a given length.
No prior art is known which discloses changing the tape feed speed adaptively during operation upon the occurrence of a predetermined condition, such as interblock gap overruns (backhitches), for the purpose of optimizing tape drive performance to best suit the operational mode or capability of the host system. U.S. Pat. No. 3,582,568 discloses an audiotape reproducing system wherein tape is normally fed at a nominal rate; but if the user wishes to skip from one musical selection recorded on the tape to a subsequent musical selection, he depresses a button that initiates a high speed advance of the tape until the head reaches the nonrecorded gap between selections. At that point it slows down in time to start playing the next selection at the nominal speed. This process can be repeated to skip unwanted intervening selections. Thus, the speed change is initiated by the operator.
U.S. Pat. No. 3,586,789 discloses apparatus wherein, during recording or playback of a block of information signals, the tape normally is fed at low speed; but between blocks of information, the tape is fed at high speed, speed being reduced just before the next block of data is to be processed. This apparatus employs signals recorded on a control track coextensively with information signals recorded on an information track.
U.S. Pat. No. 3,596,007 discloses an apparatus for reading tape on which a plurality of record tracks have been recorded at different speeds. Each record track consists of one channel carrying message information or data and a second channel having a predetermined constant frequency control signal recorded thereon to control the output voltage to a motor so that it will drive the tape at the appropriate speed called for by the frequency control signal. Here, again, the speed is controlled by indicia prerecorded on the tape.
U.S. Pat. No. 3,684,298 discloses a capstan-type tape drive wherein rotational speed of the capstan and hence the lineal speed of a magnetic tape can be selected manually by an operator.
U.S. Pat. No. 3,364,495 broadly discloses apparatus wherein the interblock gap is effectively maintained within a preselected range during writing and access time to the next block is improved if a write command is received within a preselected time period.